This invention relates to electroplating metal elements.
In microelectronic devices there are semi-conductor components with integrated circuits formed on a substrate, often of leaded glass. The circuits or chips, as they are often called, have metallic elements, terminals or leads, which leave the integrated circuit for connection to other circuitry. These terminals are conventionally formed as spaced parallel fingers and have in the past been plated with pure tin to a thickness of about 200 to 300 micro inches. The plating has included as a co-deposit a brightner such as Janus Green, or a product 6487-Igepal from Dow Chemicals, or Schlotter Tin, a product available through Learonal Corporation in the United States.
These organic brightners provide a uniform flowability to the tin and also provide a better brighter cosmetic finish. As such these terminals permit the soldering of very fine wires. Without such brightners the appearance of the terminals will be matt-like and the performance of the terminals in so far as the uniform flowability will be reduced.
In practice it has been found with such terminals that whiskers or dendrites gradually grow between the parallel terminals as a result of electrical current passing through these terminals. Over time the dendrites form a short circuit between these terminals with consequent serious results, especially where the microelectronic device is involved in high technology equipment such as spacecraft, computers, aircraft and the like.
As a result of the falure rate with these known components, the United States government has recently introduced higher specifications for the components to be applied in military applications. This requires that the plating no longer include the organic brightners of the kind mentioned. The specifications, however, do call for a product having sufficient metal flow of the terminals that solderability and corrosion resistance requirements are achieved. Preferably the product should also have the cosmetic brightness characteristics of known products having brighteners. Specifically, the specification calls for the tin plate to be between 200 to 800 micro inches (5.08 to 20.32 micro millimeters) thick. It should also be dense, homogenous, continuous and free of co-deposited organic material. Bright acid tin plate is prohibited.
One solution to obtain the improved reflow characteristics of such tin plating is to use organic and inorganic fluxes; however, this is generally undesirable since the fluxes normally contain chlorides which attack the leaded glass substrate.
Another prepared solution to the above difficulties is the addition of lead to the plating composition. This prevents the growth of whiskers and dendrites.
One method of applying a tin-lead composition to the terminals is to dip a tinned terminal after electroplating into a hot solder dip of tin and lead mixture so as to obtain an eutectic coating on the terminals. A disadvantage with this approach is that the hot dip tends to break the glass substrate as a result of the sudden temperature change.
A possible solution is the co-deposit of tin and lead during electroplating. Where tin-lead plating as a co-deposit is used as an alternative to tin plating the Government specificiations require that the lead proportion shall be 2% to 50% by weight and it should be homogeneously co-deposited.
However, electroplating with tin and lead in a chloride or borate solution is not feasible because chlorides and borates attack leaded glass. Further lead is not soluble in a sulfate plating solution.
Thus, there is a need for a method to plate a metallic lead or terminal of an integrated circuit having a leaded glass component with a plating composition that avoids dendrite growth, that can be soldered, and is corrosion resistant.